1. Field of the Invention
The present invention relates to a method of analysing drilling fluids and is particularly useful for the determination of solids in drilling fluids and differentiation between the different types of these solids.
2. Description of the Related Art
During the drilling of an oil or gas well, large volumes of solids cuttings are generated by the drill bit. These solids are conveyed to the surface by means of circulating drilling fluids, often known as "mud", which are often complicated mixtures of water, oil, soluble and insoluble minerals, polymers, surfactants and salts. A large portion of the drill cuttings can usually be removed from the fluid by means of vibrating screens and other solids separators, so that after this cleaning process the fluid can be recirculated down the drill string. However, some of the drill cuttings are dispersed as very small particles, which cannot be removed by the solids separation process. The presence of these fines, often called "drilled solids" (DS), affects the functional properties of the fluid. Determination of the concentration of DS is particularly complicated by the presence of two other types of solids in the fluid, commercial clays and weighting material.
Commercial clays are added to the fluid for control of rheological and filtration properties. Usually, bentonite or other chemically treated clay minerals are added for this purpose. The total of commercial clays (CC) and DS is called "low gravity solids" (LGS).
Weighting materials are used to bring the fluid to the required density, necessary to contain underground formation fluids by hydrostatic pressure exerted by the mud column in the annulus. A common weighting material is powdered barite (barium sulfate). The concentration of weighting materials is known as "high gravity solids" (HGS).
It is important for effective control of the properties of the fluid to know the individual concentrations of all types of solids. In current well site technology LGS and HGS are not measured directly, but are calculated from the density and solids volume fraction of the drilling fluid, both of which can be measured. The principle of the calculation is that both properties are functions of the volume fractions of LGS and HGS. A simple form of these functions is assumed, and from the resulting set of two simultaneous equations with two unknowns one can readily solve LGS and HGS. This is usually done by direct calculation or by the use of charts which may allow for corrections if salts are present.
The total concentration of clays can conveniently be determined at the well site from their ability to absorb certain cationic dyes. This is the basis for the well-known methylene blue test, which provides a value known as "MBT value" for the concentration of chemically active clays. The larger part of these clays comes usually from the intentionally added commercial clays, the remainder is derived from clay minerals in the drilled solid fraction. If an average proportion of chemically active clays in the drilled solids is assumed, then it is possible to calculate DS and CC from LGS and MBT.
As mentioned, previously proposed techniques for the determination of individual solids concentrations relies on the direct measurement of density and solids volume fraction of the fluid. Density is measured with a "mud balance", a well-known rig site device. Solids volume fraction is measured by evaporation of a fluid sample of known volume in an electrically heated distillation apparatus, also well-known as the "mud retort". The liquid distillate is collected in a graduated receiver, and from the volume of distillate and the volume of the original fluid sample the volume fraction of total solids is calculated. Problems associated with the use of these two pieces of equipment are the possibility of an incorrect apparent density as determined by the mud balance because of the presence of gas in the fluid sample, and further, the fact that the mud retort suffers from errors caused by leaks in the vapour condenser and the inability to accurately introduce a known sample volume. Further errors are also caused by the assumptions enabling solution of the simultaneous equations mentioned previously. In this respect a density for the weighting material needs to be assumed. In the case of barite, drilling grade material is in fact a mixture of pure BaSO4 (specific gravity 4.5) and impurities. The commercial product has specific gravity 4.20-4.25 at best. Furthermore, the specific gravity of the drilled solids are known with even less certainty. Corrections need to be applied if oil or soluble salts are present in the fluid. These corrections are also based on measurements with limited reliability and on not precisely known physical properties. Because on the large number of measurements and questionable assumptions made in the current methods, the resulting values for DS and HGS are known to be of limited value. It is, for example, not unusual to find negative numbers for the concentration of barite, or to find unreasonably high values for the drilled solids content.
Control of the composition has been recognised as important in maintaining the desired properties of the drilling fluid and various methods have been proposed for monitoring the composition of the drilling fluid. The ionic composition of the fluid can be analysed using ion chromatography and this can also be used to obtain an indication of the cation exchange capacity of the clay materials present. Examples of this are found in our copending patents and applications U.S. Pat. No. 4,904,603, U.S. Pat. No. 4,878,382 and EP 89203062.8 which are incorporated herein by reference. It has also been found that a more complete analysis, including information on both organic and inorganic and mineral composition can be obtained using FFIR. Examples of this can be found in our copending European and UK patent applications EP 90202796.0, EP 90202795.2, and
UK 9107041.7 also incorporated herein by reference. However, all of these methods encounter some problem when attempting to determine the amount of solids in a sample, particularly if the size of the solid particles interferes with the measurements.